Urban train headlights

[Jurek]

Why do urban trains have one LED headlight and the other of the incandescent variety?
Where's Rower40 when you need him?

[Sergeant Pluck]

I think there is a difference in brightness rather than bulb type. There’s an interesting Reddit post on the subject albeit mostly related to the Olden Days.

https://www.reddit.com/r/trainsimworld/comments/wjqa5o/comment/ijl5f5w/

Historically, the arrangement of headlamps was varied to indicate the type of train being hauled. This was useful to signalmen, who could often use it to decide what priority and direction to signal that train with, and send the information forward to the next box accordingly.

A local passenger train might carry a single light top-centre, a semi-fast two lights laterally on the buffer-beam, and a named express would combine the two in a triangle. Meanwhile a freight train would combine a low-centre light with one of the other three positions, eg. low and high centre lights would be an express freight. The Southern Region further complicated this with two additional lamp positions on the smokebox door, due to their more complicated routing.

While primarily a steam-era practice, lamp headcodes survived into the early days of diesel and electric traction, with early batches of such locomotives being fitted with headcode discs and lights in the same arrangement as the lamp-irons on most steam locomotives. Subsequently these lamp arrangements were replaced by four-figure roller-blind headcode displays, and still later these too were removed with the spreading practice of integrating four-figure train describers into signalboxes.

In all cases these "marker" lights were not bright enough to illuminate the track ahead. More powerful headlamps only began to be fitted (typically a single lamp near the centre of a locomotive end) when the headcode displays were removed. This was around the time that the second-generation DMUs (ie. Pacer and Sprinter series) were being introduced, and these were fitted with "Group Standard" headlight clusters comprising a marker light, a headlight, and a tail light on each side at each end. The marker lights resembled the old "semi-fast passenger" light code, and there were no lights fitted on the centreline.

The provision of more powerful headlamps raised the prospect of being able to see the track for a short distance ahead and even reading lineside signs at night, especially with the introduction of retroreflective signs (following road practice). The more powerful lights also made a train's presence significantly easier to notice from a distance during the day, which was important for the safety of track workers. This arrangement of headlights subsequently became standard on EMUs and locomotives built from then onwards, but was not usually retrofitted to older locomotives and units (with the notable exception of some Class 37s).

It is these differing benefits of headlights at night versus in daylight which led to only one of the two powerful headlights being used at a time, while the marker lights on both sides are lit at all times. The offside (right-hand) light is closer to the centre of a double or quadruple track formation, and thus maximises the distance at which it can be seen by track workers around curves, hence it is used in daylight. The nearside (left-hand) light is closer to lineside signs and the driver, so it is used at night to maximise its utility at that time. The provision of two independent headlamps also means that failure of one will be noticed immediately (and thus scheduled for maintenance attention) but can be substituted with the other headlamp with only a minor deficit in intended performance.

[Kim]

Where's Rower40 when you need him?

Tradition would be to page him to the diagonally striped courtesy phone...

[Cudzoziemiec]

I'm not sure about urban trains specifically, but some have a day headlight on one side and night on the other, with the night headlight being set up so that trackside signs reflect back to the driver, so presumably on the left. Not sure about light sources but presumably the day light is also brighter.

[aidan.f]

 SP's post has made me appreciate the amount of skill and knowlege  drivers and firemen had. Even today a highly skilled job.

[rogerzilla]

Freight trains require a huge amount of skill.  Besides being half a mile long (meaning they take a long time to clear junctions - drivers set a train length indicator to tell them when the last wagon has cleared)and enormously hard to accelerate, they can play tricks due to momentum and springing, storing energy and giving the loco a hard shove.

It was, of course, worse in the old days when the only brakes available when in motion were on the loco and a single brake van, and couplings were all loose 3-link chains.  That's why they had a maximum speed of 25mph.  Even then,couplings would break if the driver and guard allowed them to become slack, then snatched them tight.

[mark]

Freight trains require a huge amount of skill.  Besides being half a mile long (meaning they take a long time to clear junctions - drivers set a train length indicator to tell them when the last wagon has cleared)and enormously hard to accelerate, they can play tricks due to momentum and springing, storing energy and giving the loco a hard shove.

It was, of course, worse in the old days when the only brakes available when in motion were on the loco and a single brake van, and couplings were all loose 3-link chains.  That's why they had a maximum speed of 25mph.  Even then,couplings would break if the driver and guard allowed them to become slack, then snatched them tight.

I've measured trains around here at 1.3 miles long while driving past them. Are they shorter on your side of the pond?

[rafletcher]

Much. A quick google says 775m is the longest, while more usually 500m.

[rogerzilla]

Much. A quick google says 775m is the longest, while more usually 500m.

Most freight flows are 22 bogie wagons (count them next time you see one go past).  Drax biomass hoppers, which are on the longer side, are 18.8m.  So 22 of those makes 413m.  Add about the same for the loco and it's about 500m.

The 775m trains are probably full of something really light.

[Jurek]

Meanwhile, back at the headlights, the one which is white LED at the front on the nearside of the train, has an identical one on the offside, but this isn't used. Not until, that is, it is at the back of the train and then they both shine in red LED.
I know it to be possible to change the colour of some LEDs by changing the amount current or voles, I forget which, you feed to them.
The nearside incandescent headlamp doesn't appear to be used in any configuration.
Also, why on The London Overground, are there buttons to close the doors?
There is no time during which these buttons are active.
That's a lot of buttons and (presumably) wiring on each train.
For what purpose?

[Kim]

Meanwhile, back at the headlights, the one which is white LED at the front on the nearside of the train, has an identical one on the offside, but this isn't used. Not until, that is, it is at the back of the train and then they both shine in red LED.
I know it to be possible to change the colour of some LEDs by changing the amount current or voles, I forget which, you feed to them.

Not really; while white LEDs (which are just blue LEDs with multiple phosphors) can go a bit wibbly in their colour output at low brightnesses, proper colour-changing LEDs are usually more than one diode in the same package.  The hated bi-colour LEDs found on battery chargers and the like are a red and a green back to back with opposite polarity, so you can make them red by passing current one way, green by passing current the other way, or yellow by feeding them AC.  More fancy ones do RGB, either with separate pins for each diode or by including a small chip that drives the diodes at brightnesses set by a serial data stream.  Lighting-oriented LEDs often come in RGB+W (because mixing RGB to get white doesn't work as well as you'd hope) or even RGB-CW-WW so the colour temperature can be adjusted, or just CW+WW if you don't need colour.

I suspect the train headlights are dedicated units made up of either individual white and red sources, or off-the-shelf RW or RGBW modules.  They'll have some controller that speaks CANBUS or whatever trains use.

Also, why on The London Overground, are there buttons to close the doors?
There is no time during which these buttons are active.
That's a lot of buttons and (presumably) wiring on each train.
For what purpose?

Possibly just for when the train's stood at a platform with the doors unlocked for 20 minutes, so the passengers don't freeze?  That may or may not be a feature of a given timetable, of course.

Or perhaps they're used by the guard when in manual mode?

[Cudzoziemiec]

On some trains it is possible to have the headlights and tail lights on at the same time, at the same end. This results in a sort of pink glow and train drivers have a special sign they make when to tell other drivers this is what's happened ("sort it out before the boss sees" kind of thing). I expect however these are older, pre-LED units.

[grams]

Much. A quick google says 775m is the longest, while more usually 500m.

The limitation on train length is sidings they can be parked in while passenger trains overtake them, plus those at the origin and destination.

Possibly just for when the train's stood at a platform with the doors unlocked for 20 minutes, so the passengers don't freeze?  That may or may not be a feature of a given timetable, of course.

That's exactly what they're for. Although modern babbage-enabled trains will close the doors automatically after a certain time period.

Or perhaps they're used by the guard when in manual mode?

The what? London Overground doesn't employ any.

[ian]

In olden times you used to have to open the doors on the overground. Probably to stop you falling out at New Cross Gate. Then it went automatic, not sure why.

[Tim Hall]

They'll have some controller that speaks CANBUS or whatever trains use.

There's two systems, one for use during the week, whose name escapes me, and one at weekends,  CAN-Rail replacement-BUS.

[SteveC]

In olden times you used to have to open the doors on the overground. Probably to stop you falling out at New Cross Gate. Then it went automatic, not sure why.

I'd guess for speed. The bottleneck on the Underground is now moving people on and off the platforms, not packing more trains in.

[rogerzilla]

The Japanese solved that decades ago, although I'm not sure British people are yet ready to be physically shoved by soneone who looks like one of Viz's The Bottom Inspectors in white gloves.